How are the fish being acclimated?
Also, are these fish all coming from the same source? If so, I'd check their
specific gravity. I've heard of LFS
using hyposalinity on an ongoing basis to try and control ich. The problem comes sometimes when you bring the fish home. Apparently, fish can take a downward move in salinity prety well, but can't adapt to an upward move nearly as fast.
Finally, I would strongly advise you to invest in a quarantine tank and place new fish in quarantine for a minimum of two weeks. (Longer is almost always better.) That way, you can make sure they eat and get settled in before going into the main display. If there is a disease breakout, you can treat it in quarantine, away from your established fish and inverts. Almost all of the proven effective disease treatments out there will kill inverts. There are a lot of people out there (myself included) who learned this the hard way.